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Testosterone

Testosterone concentrations in the stallion vary with season, with a mean of 3 nmol/l in the non-breeding season rising to 12 nmol/l at the height of the breeding season. Geldings have low or undetectable concentrations (<0.19 nmol/l) of testosterone. A cryptorchid is confirmed if the serum testosterone concentration is greater than 0.30 nmol/l.

Baseline testosterone concentrations are not significantly different between stallions and unilateral cryptorchid horses. Bilaterally cryptorchid horses have a lower baseline testosterone concentration and most commercially available assays are not sensitive enough to detect these low levels. A single serum testosterone test is therefore NOT recommended for diagnosing cryptorchidism.

The equine testis produces ten times the quantity of oestrogen compared with testosterone and it is therefore much easier to differentiate a cryptorchid from a gelding by measuring serum oestrone sulphate.

This test is not recommended in horses less than 3 years of age and donkeys. These younger animals require measurement of serum testosterone after stimulation with hCG. A protocol for this test is as follows:

  • Take a baseline blood sample.
  • Inject 6000 IU hCG IB
  • Collect blood between 30 minutes ad 2 hours after hCG

A true cryptorchid should show an increase in testosterone from 0.3 – 4.3 nmol/L to 1 – 12.9 nmol/L. Castrated horses should remain below 0.19 nmol/L.

Alpacas and Llamas

A single testosterone measurement is often all that is required. If equivocal results are found, then an HCG stimulation test can be used. For this test, serum testosterone concentrations are measured in two blood samples collected before and 18 hours after IM administration of 5000 IU HCG. NB: Alpacas reach sexual maturity at 10-12 months of age and llamas at 20- 24 months.

Species:

Equine, alpaca

Specimen:

Serum

Container:

Plain (red top) or gel tube

Collection protocol:

Standard venepuncture.

Special handling/shipping requirements:

Separate the serum and transport to the laboratory within 12-24 hrs.